1. Field of the Invention
The present invention relates to a movable fiber optical switch and, more particularly, to an m- input/n- output (mxn; e.g., 1.times.2, 2.times.1 or 2.times.2) optical fiber switch.
2. Description of the Prior Art
In evolving optical communications networks, the need often arises to switch an optical signal from one path to another. For example, in a network which consists of a number of communication nodes connected sequentially to form a ring, it may be required to temporarily remove one or more nodes from the network. Therefore, the optical fibers interconnecting the network must be able to "switch" the node(s) from an active (transmit/receive) state to a passive (bypass) state. Many arrangements exist in the art for providing this type of switching. The arrangements may be generally classified into two groups: (1) moving beam switches (arrangements which redirect the optical signal path between stationary fibers); and (2) moving fiber switches (designs which use an external force to physically change the location of the fibers entering and exiting the node). The switch of the present invention falls into the latter category.
There are many arrangements in the prior art for moving an optical fiber so as to effect this switching function. Most of the arrangements utilize the presence or absence of an applied magnetic field to perform the switching. One such device is disclosed in U.S. Pat. No. 4,204,744 issued to J. Wittmann on May 27, 1980. The Wittmann structure is referred to as an one input/two output (1.times.2) switch, with each fiber being held in place by a V-groove appropriately positioned on a substrate. The substrate is further formed to contain a recessed cavity between the input V-groove and a pair of output V-grooves. A pair of optical fibers are permanently attached to the output V-grooves. An input fiber is placed in the input V-groove and is positioned to extend into the cavity so that the end of the input fiber is free to traverse the cavity from one output V-groove location to the other. A magnetic sleeve fits over the free end of the fiber. An appropriately applied magnetic field will thus cause the fiber to move from one edge of the cavity to the other so as to align it with the chosen output fiber. In this arrangement, alignment is controlled by the ability to accurately position the edges of the cavity with respect to the output V-grooves, but this alignment may prove difficult in some circumstances. Additionally, when this type of 1.times.2 switch is used in a system for the switching of four fibers, two such switches must be used in tandem. In an optical configuration, the use of two such switches causes a significant increase in the insertion loss at each node, as well adding expense in terms of fiber management.
An alternative arrangement which addresses the latter concern is disclosed in U.S. Pat. No. 4,337,995 issued to T. P. Tanaka et al. on July 6, 1982. This configuration utilizes four separate optical fibers which enter one switch housing, with two fibers being movable within the housing. In the active (transmit/receive) state, all the fibers are stationary; the end-to-end coupling between pairs of fibers allowing for the node to communicate with the ring. Alternatively, when the switching is activated by an applied magnetic field, a pair of fibers move to alternate locations, allowing for the ring to bypass the node.
A problem with both of the discussed arrangements is that the designs are capable of providing only one input/two output (1.times.2) switching. This limitation is considered to be significant, since situations exist where it would be desirable to perform a (2.times.2) switching operation. For example, when a particular node is bypassed, an interconnection of the transmitter's output fiber to the receiver's input fiber would allow for self-testing and stand-alone operations to continue to be performed.
Thus, a need remains in the prior art for an optical fiber switch which is capable of performing a (2.times.2) switching function in addition to a (1.times.2) switching function.